The replacement of doped polycrystalline silicon structures with a two-layer structure comprising a layer of a refractory metal silicide overlying a layer of doped polycrystalline silicon to obtain improved lateral conductivity is becoming increasingly common in the electronics industry. Such structures are commonly termed "polycide" structures. The sheet resistance of polycide structures is generally an order of magnitude lower than that of a single layer structure of doped polycrystalline silicone alone.
Polycide structures are utilized to achieve a high-conductivity, gate-level metallization for metal-oxide-semiconductor (MOS) devices. More particularly, polycide structures are utilized as gate material and interconnects in structures, such as very high speed integrated circuits (VHSIC). Polycide structures have been used, for example, as the gate material in insulated gate field effect transistor (IGFET) devices and other similar structures.
Polycide structures are conventionally produced by depositing undoped polycrystalline silicon by low pressure chemical vapor deposition (LPCVD), doping the polycrystalline silicon, e.g. by diffusion using phosphorus oxychloride, and then depositing the refractory metal silicide, e.g. tantalum silicide, thereover by co-sputtering from separate targets of metal and silicon. Subsequent annealing is required to obtain the desired low conductivity of the silicide layer. This method, while effective, has the disadvantage that it must be carried out sequentially in different types of apparatus which involves multistep handling of the substrate.
Lehrer, in U.S. Pat. No. 4,359,490, discloses a process of forming polycide structures entirely by LPCVD. The ratio of tantalum to silicon in the as-deposited film is not given, except that it is referred to as "tantalum silicide". Lehrer et al., in a later paper presented at the First International Symposium on VLSI Science and Technology, October 1982, describe depositing tantalum silicide directly from tantalum pentachloride and silane at temperatures of 600.degree. to 650.degree. C. The film is stated to be metal rich, i.e. close to Ta.sub.5 Si.sub.3. It is noted, however, that the upper surface of the silicide layer and the interface between the silicide and the polycrystalline silicon layers became quite rough in converting the metal rich silicide to stoichiometric TaSi.sub.2 due to the nonuniform adsorption of silicon from the silicon layer. It was therefore concluded by Lehrer et al. that it would be desirable to deposit tantalum silicide directly in the form of stoichiometric TaSi.sub.2.
In accordance with this invention, a method of forming polycrystalline silicon/tantalum silicide structures has been found in which all steps are carried out in a single reaction chamber and which does not suffer the disadvantages described above.